1. Field of the Invention
This invention relates to a variable optical-property element capable of changing optical properties, such as a variable focal-length lens, variable focal-length diffraction optical element, variable deflection-angle prism, or variable focal-length mirror, and in particular, to an optical apparatus including the variable optical-property element.
2. Description of Related Art
The focusing operation of a zoom lens system or an imaging device is usually performed by mechanically moving lenses. However, for an electronic endoscope which is required to be subminiature or the eye of a micromachine, it is impossible to move the whole or a part of a lens system. Moreover, it is desirable that a TV camera, an electronic still camera, or a silver halide film camera is capable of performing zooming and focusing operations without moving the lens system in order to achieve its compactness and reduction in cost.
As means for performing the zooming and focusing operations without moving the lens system, variable focal-length lenses have been proposed, for example, in Japanese Patent Preliminary Publication Nos. Hei 5-34656 and Hei 4-345124.
FIG. 1 shows a liquid crystal lens which is an example of this type of variable focal-length lens. In this figure, reference numeral 1 represents a nematic liquid crystal with molecules obliquely oriented which is hermetically sealed by a seal member 2 and sandwiched between a pair of transparent substrates 4 and 5, each having a doughnut-shaped electrode 3. Reference numeral 6 denotes orientation films provided on surfaces inside the transparent substrates 4 and 5; 7 denotes a polarizing plate placed on the front (left) side of the substrate 4, transmitting only light vibrating in the plane of the paper; and 8 denotes an AC power supply connected to the electrodes 3 through a switch 9 and a variable resistor 10.
In this liquid crystal lens, when the switch 9 is turned off, the molecules of the liquid crystal 1, as shown in the figure, are obliquely oriented so that light rays L travel in straight lines. In contrast to this, when the switch 9 is turned on to apply voltages to the electrodes 3, the direction of an electric field becomes uneven because the electrodes 3 are doughnut-shaped, and the molecules of the liquid crystal 1 are oriented as shown in FIG. 2. Specifically, the molecules of the liquid crystal 1 maintain an oblique orientation in the vicinity of the center of the liquid crystal lens in which the strength of the electric field is diminished. However, since the strength of the electric field is increased progressively in approaching the electrodes 3, the molecules of the liquid crystal 1 are oriented perpendicular to the substrates 4 and 5. Hence, for polarized light transmitted through the polarizing plate 7, the refractive index of the liquid crystal 1 becomes high in going from the periphery to the center of the liquid crystal lens so as to have an index distribution in a radial (y) direction of the liquid crystal lens. In this way, the liquid crystal lens becomes an inhomogeneous lens having the function of a positive lens, and thus the light rays L of incidence converge.
However, this conventional liquid crystal lens, which needs the polarizing plate 7, has the drawback that the amount of transmitted light is so small that a transmittance is as low as 30-40%, and applicable products are highly limited.
Furthermore, a conventional variable focal-length lens has a mechanically complicated structure that because a lens made by grinding glass is used and the focal length cannot be changed by the lens itself, a part of a lens unit must be moved along the optical axis as in the zoom lens of a camera to change the focal length.
In order to obviate such a defect, it is necessary to change the focal length of the lens itself, and as shown in FIG. 3, an optical system using a polarizing plate 11 and a liquid crystal lens 12 is proposed. The liquid crystal lens 12 used in this optical system has lenses 13a and 13b and a liquid crystal layer 15 sandwiched through transparent electrodes 14a and 14b between the lenses 13a and 13b. The liquid crystal lens 12 is designed so that an AC power supply 17 is connected through a switch 16 between the transparent electrodes 14a and 14b to selectively apply the electric field to the liquid crystal layer 15, and thereby the refractive index of the liquid crystal layer 15 is changed.
In the optical system including the polarizing plate 11 and the liquid crystal lens 12 as shown in FIG. 3, for example, when natural light is rendered incident on the optical system, only a predetermined linearly polarized light component is transmitted through the polarizing plate 11 and enters the liquid crystal lens 12.
Here, as show in FIG. 3, when the switch 16 is turned off and the electric field is not applied to the liquid crystal layer 15 of the liquid crystal lens 12, the major axes of liquid crystal molecules 15a point in the same direction as in the linearly polarized light component, and thus the refractive index of the liquid crystal layer 15 is increased. Consequently, the focal length of the liquid crystal lens 12 is diminished.
In contrast to this, as shown in FIG. 4, when the switch 16 is turned on and the electric field is applied to the liquid crystal layer 15, the liquid crystal molecules 15a is such that since their major axes become parallel to the optical axis, the refractive index of the liquid crystal layer 15 is lowered and the focal length of the liquid crystal 12 is increased.
In this way, the optical system shown in FIGS. 3 and 4 is constructed so that the electric field is selectively applied to the liquid crystal lens 12 and thereby the focal length is changed.
However, this optical system requires that the polarizing plate 11 is placed on the front side of the liquid crystal lens 12 to render only the predetermined linearly polarized light component incident on the liquid crystal lens 12. Hence, the optical system has the disadvantage that the amount of light transmitted through the polarizing plate 11 to enter the liquid crystal lens 12 is reduced and the efficiency of use of light is impaired. Consequently, there is an additional disadvantage that products to which the optical system is applicable are limited and its versatility is lost. Furthermore, there is another disadvantage that much time is required to change the focal length.
On the other hand, the variable optical-property element, such as a liquid crystal lens, has the advantage that optical properties such as a focal length, can be changed by a single optical element. However, the use of only the variable optical-property element, which causes spherical aberration, distortion, and chromatic aberration, is unfavorable.
In addition, the variable optical-property element has the drawback that when its optical properties, for example, the focal length is changed, aberration fluctuates or flare-increases.
In the optical system of the variable optical-property element, a free-formed surface optical element may be used. The free-formed surface of the free-formed surface optical element refers to a curved surface composed of an irrotational symmetric surface, which may or may not include one symmetric surface. A surface in which a rotational symmetric surface is decentered also comes under the class of the free-formed surface. An optical system using the optical element with the free-formed surface (irrotational symmetric surface) utilizes the reflection of the free-formed surface, and thus has the merit that chromatic aberration is not produced. This optical system, however, has the disadvantage that the shape of the curved surface is abnormal, and thus when the optical element is moved for the zooming and focusing operations, a mechanical structure such as a moving mechanism becomes complicated.